Steel is a metallic alloy that can have exceptional strength characteristics, and which is accordingly commonly utilized in structures where strength is required or advantageous. Steel can be utilized, for example, in the skeletal supports of building structures, tools, engine components, and protective shielding of modern armaments.
The composition of steel varies depending on the application of the alloy. For purposes of interpreting this disclosure and the claims that follow, “steel” is defined as any iron-based alloy in which no other single element (besides iron) is present in excess of 30 weight percent, and for which the iron content amounts to at least 55 weight percent, and carbon is limited to a maximum of 2 weight percent. In addition to iron, steel alloys can incorporate, for example, manganese, nickel, chromium, molybdenum, and/or vanadium. Accordingly, steel typically contains small amounts of phosphorus, carbon, sulfur and silicon.
Steel comprises regular arrangements of atoms, with the periodic stacking arrangements forming three-dimensional lattices that define the internal structure of the steel. The internal structure (sometimes called “microstructure”) of conventional steel alloys is always metallic and polycrystalline (consisting of many crystalline grains). Both composition and processing methods are important factors that effect the structure and properties of a steel material. In conventional steel processing, an increase in hardness can be accompanied by a corresponding decrease in toughness. Steel material produced by conventional methods that increase the hardness of the composition can result in a steel material that is very brittle.
Steel is typically formed by cooling a molten alloy. For conventional steel alloys, the rate of cooling will determine whether the alloy cools to form an internal structure that predominately comprises crystalline grains or, in rare cases, a structure that is predominately amorphous (a so called metallic glass). Generally, it is found that if the cooling proceeds slowly (i.e., at a rate less that about 104 K/s), large grain sizes occur, while if the cooling proceeds rapidly (i.e., at rate greater than or equal to about 104 K/s) microcrystalline internal grain structures are formed, or, in specific rare cases not found in conventional steel alloy compositions, an amorphous metallic glass is formed. The particular composition of a molten alloy generally determines whether the alloy solidifies to form microcrystalline grain structures or an amorphous glass when the alloy is cooled rapidly.
Both microcrystalline grain internal structures and metallic glass internal structures can have properties that are desirable in particular applications for steel. In some applications, the amorphous character of metallic glass can provide desired properties. For instance, some glasses can have exceptionally high strength and hardness. In other applications, the particular properties of microcrystalline grain structures are preferred. Frequently, if the properties of a grain structure are preferred, such properties will be improved by decreasing the grain size. For instance, desired properties of microcrystalline grains (i.e., grains having a size on the order of 10−6 meters) can frequently be improved by reducing the grain size to that of nanocrystalline grains (i.e., grains having a size on the order of 10−9 meters). It is generally more problematic, and not generally possible utilizing conventional approaches, to form grains of nanocrystalline grain size than it is to form grains of microcrystalline grain size.
It is desirable to develop improved methods for forming nanocrystalline grain size steel materials. Further, as it is frequently desired to have metallic glass structures, it is desirable to develop methods of forming metallic glasses. Still further, it is desirable to develop methods of processing steel that can achieve an increased hardness without a corresponding loss of toughness.